In general, a thermal recording sheet is obtained by mixing a normally colorless or pale colored dye precursor and a color developer such as a phenolic compound, each dispersed to fine particles and mixed, adding a binder, a filler, a sensitizer, a slip agent, and other additives to form a coating color, and coating the coating color on a substrate such as paper, synthetic paper, films, or plastics, which develops a color by a momentary chemical reaction caused by heating with a thermal head, a hot stamp, a thermal pen, laser light or the like to obtain a recorded image.
Thermal recording sheets are applied in a wide variety of areas such as measuring recorders, terminal printers for computers, facsimiles, automatic ticket venders, and bar code labels. However, with recent diversification of these recording devices and advance towards higher performance, quality requirements for thermal recording sheet have become higher and more difficult to achieve. For example, for high-speed recording, a thermal recording sheet which can provide a high recording density and a sharp recorded image even with a small thermal energy is in demand. On the other hand, in view of storage stability of the recording sheet, the thermal recording sheet is required to be superior in light resistance, weather resistance, and oil resistance.
Examples of conventional thermal recording sheets include, for example, a thermal recording material disclosed in Japanese Patent Publication 43-4160 or 45-14039. However, such conventional thermal recording materials have been low in thermal response and insufficient in color developing density when used in high-speed recording.
As methods for eliminating such disadvantages, high-sensitivity dyestuffs have been developed such as use of 3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluorane (Japanese Patent Laid-open Publication (OPI) 49-109120) or 3-dibutylamino-6-methyl-7-anilinofluorane (Japanese OPI 59-190891) as a leuco dye, and as a color developer, use of 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane (Japanese OPI 59-106456), 1,5-bis(4-hydroxyphenylthio)-3-oxaheptane (Japanese OPI 59-116262), or 4-hydroxy-4'-isopropoxydiphenylsulfone (Japanese Patent Publication 63-46067) to achieve high-speed recording and high sensitivity.
However, although these thermal recording sheets have been high in sensitivity, have had a problem in thermal resistance in that when stored at high temperatures, they tend to result in a reduction of image density.
Further, since they are considerably low in storage stability of the recorded image, they still have disadvantages in that a considerable decrease in the image density or discoloration occurs when water or a sebaceous matter contacts the color image or the image contacts with a plasticizer (DOP, DOA, or the like) contained in wrap films such as PVC films.
Therefore, a primary object of the present invention is to provide a novel urea (thiourea) derivative and a thermal recording sheet with a high sensitivity and superior in thermal resistance, water resistance, and oil resistance by using the compound.